A comparison between different Eddy Current excitation methods for the inspection of thin metallic parts and their robustness against Lift-Off invariance

Speaker:Ricci, Marco; University of Calabria; Italy

Authors:Ferrigno, L.; University of Cassino and South Lazio; ItalyLaracca, M.; University of Cassino and South Lazio; ItalyMalekmohammadi, H.; University of Perugia; ItalyRasile, A.; University of Cassino and South Lazio; ItalyRicci, M.; Università della Calabria; ItalySilipigny, G.; University of Perugia; Italy

Eddy Current Non-destructive testing technique is one of the most widely used techniques in many areas of industry. There are lots of factors involved in measurements which may affect the detection of defects in metallic (and recently non-metallic e.g. CFRP) parts and structures. One key factor is the Lift-off, whose variation is difficult to avoid in on-site measurement process. To face this issue, various strategies have been developed to reduce or eliminate the effect of lift-off by using various excitation strategies such as Multi-Frequency, Pulsed and Swept Frequency Eddy Current. The aim of this paper is to compare these excitation strategies and their robustness against lift-off with specific focus on swept frequency (Chirp) excitation. The comparison will be implemented on experimental data collected on samples consisting of layers of Aluminium alloy with internal small notches. Chirp Signals have been first introduced in Radar applications in order to improve the detectability with lower power signals. Then developed into Ultrasonic application and now have been used in Eddy Current NDT as well. With respect to Pulsed Eddy-current, Chirp Signals improve the detectability of defects by increasing the Signal to Noise Ratio (SNR) due to a more energetic signal. At the same time the advantages provided by Pulsed Eddy-Current derived by the possibility of performing both time- and frequency- domain analysis are assured also by Swept-Frequency Eddy Current through the use of pulse compression.